1. Field of the Invention
This invention relates generally to the field of xDSL (x Digital Subscriber Line) line interface circuitry, and more particularly to hybrid circuitry known as 2-to-4 wire converters.
2. Description of the Background Art
Typically, xDSL modems separate transmit and receive signals to cancel noise. However, when the transmit signal (one band) leaks into the receive signal (another band), the noise degrades xDSL performance. Thus, xDSL modems often use filters to prevent this leakage between bands. However, the filters cannot be efficiently sharpened to optimally cancel noise.
Instead of filters, hybrids (2-to-4 wire converters) are used to cancel noise in xDSL line interface circuitry. However, hybrids typically cannot achieve enough noise cancellation because it is difficult to construct a circuit of resistors, capacitors, and inductors with a total impedance that closely matches the transmission line characteristic impedance. It is relatively easy to match impedances at a single specific frequency, but maintaining an impedance match over a range of frequencies is difficult.
Further, noise cancellation is difficult to achieve with a hybrid because the transmission line can only be xe2x80x9cseenxe2x80x9d through a mandatory line-coupled transformer. The transformer has less than ideal properties that alter the perception of the line impedance. The most significant properties are the magnetizing and leakage inductances, which significantly degrade noise cancellation. In practice, it is not practical to produce transformers with sufficiently small leakage inductance or sufficiently large magnetizing inductance to achieve acceptable noise cancellation.
One method to improve noise cancellation is to use inductors to incorporate compensating inductances into the impedance of the hybrid. However, this method is limited by the problem of accurately matching the discreet inductor values to the inductance values intrinsic in the transformer. The inductance values intrinsic in the transformer cannot be precisely controlled and incorporating inductors does not yield optimum hybrid performance.
Therefore, what is needed is a technique that permits xDSL line interface circuitry in a communication system to cancel noise between transmit and receive signals while matching impedance on the xDSL line interface circuitry.
A system for matching impedance in the circuitry of an xDSL communication device to improve noise cancellation resulting from signal leakage between transmit and receive signals. In one embodiment, a line interface circuit includes a transformer configured to couple a matching impedance, which is substantially equal to a line impedance, to a line coupling transformer, and is applicable to all known hybrid topologies. The transformer is ideally as closely matched to the line coupling transformer as possible. This technique allows greatly improved impedance matching in the hybrid, which directly benefits the performance of xDSL communication devices.
The line coupling transformer and the other transformer are substantially identical, having substantially identical leakage inductances and substantially identical magnetizing inductances. Substantially identical transformers can be achieved using an identical manufacturing process at a single manufacturing facility. In one embodiment, the matching impedance is a complex impedance network configured to have an impedance substantially equal to the line impedance.